The present invention relates to compositions useful as adhesion promoters or primers for treating surfaces prior to bonding. In particular, these compositions contain interactive and/or compatible prepolymers and one or more adhesion promoters for improving initial adhesion and durability of the bond at the adhesive/substrate interface. The present compositions are preferably designed for use in conjunction with polyurethane adhesive compositions and have particularly enhanced adhesive promoting properties when used on metal surfaces.
Surface preparation of substrates prior to bonding is common and often required to achieve strong adhesive bonds. In addition to mechanical surface treatments, such as roughening the substrate surface, chemical treatments have been employed. These chemical treatments may modify the physical surface structure, such as by etching, anodizing or cleaning the surface, or may serve as coatings which provide a mutually compatible interface layer between the underlying substrate and the adhesive. These pre-treatment coatings serve the purpose of promoting the formation of physical and chemical intermolecular forces, thereby enhancing the strength of the adhesive bonds. For example, organosilicone compounds, either applied as a primer or incorporated into the adhesive per se, have been used to improve the strength and provide enhanced resistance to humid conditions. These adhesion promoters have been widely used in the manufacture of glass fiber-reinforced plastics to improve the adhesion between glass fibers and the matrix resin. For example, U.S. Pat. No. 4,456,718 discloses organosilicon promoters having two or three alkoxy groups and a functional organic group. These adhesion promoters, which are conventionally employed in many current applications, have the general formula: EQU (RO).sub.3 SiCH.sub.2 CH.sub.2 CH.sub.2 X
wherein R is C.sub.1-4 alkyl and x is a functional organic group such as an amino, hydroxy, vinyl, methacrylate or epoxy group. The adhesion promotion properties imparted by these compounds is believed to be due to the chemical and physical reaction of the organosilicon compounds at the interface of substrate and polymer (adhesive). Other useful adhesion promoters well known in the art include organotitanates, organic chromium and zirconium complexes.
Known adhesion promoters include isocyanate-based products having a high content of isocyanate groups, as well as organofunctional silanes and siloxanes. These materials are generally highly reactive and require special care in their preparation and storage. Many non-ferrous metals such as aluminum, copper and zinc bichromate are difficult materials on which to achieve good adhesion both initially and during subsequent exposure to heat and humidity. These conventional adhesion promoters have only limited efficacy on non-ferrous metals or metals with modified surfaces.
Adhesion promoters have been used both in primer compositions, as well as directly in the adhesive formulations. For example, abietic acids and rosin esters have been disclosed as useful tackifiers in polyurethane pressure-sensitive adhesive compositions. U.S. Pat. No. 4,037,392 discloses esters of rosin such as the pentaerythritol methyl ester, propyl ester, ethylene glycol and glycerol esters, as well as esters of hydroabietyl alcohol such as the benzoic and phthalic esters thereof, useful as tackifiers to achieve enhanced adhesion properties of polyurethane pressure-sensitive tapes.
More recently, U.S. Pat. No. 5,207,069 discloses heat-activated adhesion promoters for treating surfaces prior to bonding. These promoters are applied directly to the surface, as opposed to incorporated in the adhesive composition, and include a solvent mixture of a carbonyl compound with at least one unsaturated activated bond and a polymer containing a functional group. Examples of useful carbonyl compounds disclosed include unsaturated dicarboxylic acids and esters thereof such as esters of maleic, crotonic and fumaric acids. Those polymers disclosed include materials having carboxylic, amine or hydroxy groups such as polyesters, polyamides, polyethers, polyacrylates and polyurethanes. The promoters require heat activation in order to achieve their intended function.
There is a need for an adhesion promoter designed especially for metallic surfaces which improves the adhesion characteristics of various adhesive systems without the use of heat or additional surface treatment, which is an effective under ambient conditions and which gives rise to enhanced adhesion durability under conditions of heat and humidity.